Polymer light emitting diode (PLED) with a practically acceptable lifetime is an important issue for realization of its industrialization. Therefore, using oxygen- and moisture-stable high-work-function metals as the cathode such as Al, Cu, Ag, and Au has attracted extensive attentions recently. The crucial issue for using the high-work-function metals is the large electron-injection barrier from the metal cathode to emitting layer (EML). Water- or alcohol-soluble electron injection layer (EIL) based on hydroxyl, amino and ammonium-salt groups grafted conjugated polymer has been demonstrated to allow a use of high-work-function metal as the cathodes because the formation of interfacial dipole or space charge between EIL and the cathode can reduce electron-injection barrier (Huang, F., et al., Chem. Mater., 16, 708 (2004). Wu, H. B., et al., Adv. Mater., 16, 1826 (2004). Wu, H., et al., Org. Electron., 6, 118 (2005). Huang, F., et al., Adv. Mater., 19, 2010 (2007). Huang, F., et al., Adv. Mater., 19, 2457 (2009). Oh, S. H., et al., Adv. Mater., 20, 1624 (2008). Yang, R., et al., J. Am. Chem. Soc. 128, 14422 (2006). Seo, J. H., et al., J. Am. Chem. Soc. 130, 10042 (2008)). Consequently, the maximum external quantum efficiency ηext (and its corresponding luminous efficiency ηL) for blue, green, and red emission PLEDs based on fluorescent conjugated polymers and Al as the cathode were reported to reach 1.62% (1.3 cd/A) (Wu, H. B., et al., Adv. Mater., 16, 1826 (2004)), 7.85% (23.8 cd/A) (Wu, H. B., et al., Adv. Mater., 16, 1826 (2004)), and 2.94% (2.89 cd/A) (Huang, F., et al., Adv. Mater., 19, 2457 (2009)), respectively. But, the brightness (and applied voltage) at the ηext for the three emissions are only 380 cd/m2 (9.7 V), 7,923 cd/m2 (8.8 V), and 1,040 cd/m2 (9.4 V), respectively, which may cause the extra energy consumption and thus is detrimental to the PLED application. Therefore, enormous efforts must be made on the study of useful water- or alcohol-soluble EIL.
In addition to the hydrophilic groups above, crown ether groups may be expected to serve the same purpose. Polyfluorene grafted with 15-crown-4 moiety (PFC) is used as an EIL for poly(9,9-dihexylfluorene)-based device with Ca as the cathode to reduce turn-on voltage from 6.6 V (without PFC) to 4.1 V (with PFC) and enhance the maximum brightness Bmax (and ηL) from 880 cd/m2 (0.29 cd/A) to 2,800 cd/m2 (0.53 cd/A) due to a formation of interfacial dipole leading to a rise of vacuum level of metal cathode and thus lowering of the electron injection barrier (Yu, J. M., et al., J. Polym. Sci. Part A: Polym. Chem., 47, 2985 (2009)). Crown ethers are a special class of ethers able to form stable complexes with ions of alkali, alkaline earth, and transition metals (Pedersen, C. J., J. Am. Chem. Soc., 89, 2495 (1967). Gokel, G. W., et al., Chem. Rev., 104, 2723 (2004). Pedersen, C. J., Angew. Chem. Int. Ed. Engl., 27, 1021 (1988)). A metal ion with diameter close to the cavity diameter of a crown ether can form a stable complex with it (Pedersen, C. J., Angew. Chem. Int. Ed. Engl., 27, 1021 (1988)). For example, the cavities of 12-crown-4 (1.2-1.5 Å), 15-crown-5 (1.7-2.2 Å) and 18-crown-6 (2.6-3.2 Å) can form stable complexes with lithium ion (1.36 Å), sodium ion (1.94 Å) and potassium ion (2.66 Å), respectively (Pedersen, C. J., Angew. Chem. Int. Ed. Engl., 27, 1021 (1988)). Due to the specific chelating selectivity of crown ether on metal ion, conjugated polymers grafted with various crown ethers on side chain are widely used as a fluorescent sensor for detecting metal ions such as 15-crown-4 grafted hyperbranched (or linear) oligo(fluorene vinylene) for Ru3+ and Fe3+ ions (Yu, J. M., et al., Macromolecules, 42, 8052 (2009)) or polyfluorene derivatives with benzo-18-crown-6 pendants for Pb2+ ion (Yu, M., et al., Macromol. Rapid Commun., 28, 1333 (2007)).
So far two kinds of electron injection layers have been used for the OLEDs, which are an inorganic slat and an organic small molecule matrix doped with an inorganic salts and/or an organic small molecule. Examples of the inorganic salt are vapor deposited Ba-containing compounds such as BaF2 (Cina, S., U.S. Pat. No. 7,833,812 B2 (2010)), and metal oxides such as LiCoO2 and LiNiO2 (Qiu, Y., et al., U.S. Pat. No. 7,501,755 B2 (2009)). One example of the doped organic small molecule matrix is triazine matrix doped with N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine and SiO2 (SiOc or Sc) (Aziz, H., U.S. Pat. No. 7,111,407 B2 (2010)).